Variable evaporator control for a gas dryer

ABSTRACT

A variable evaporator control system and method in a gas dryer for maximizing the cooling which can be accomplished for a given length heat exchanger by adjusting the evaporator refrigerant approach temperature responsive to changes in the gas load on the system. Pressure and/or temperature sensors positioned at particular locations in the system provide feedback for controlling adjustments in the approach temperature depending on the gas load.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on U.S. Provisional Patent ApplicationSerial No. 60/285,528, filed Apr. 20, 2001.

BACKGROUND

[0002] The invention relates generally to gas dryers, and moreparticularly to a variable evaporator control (“VEC”) system and methodfor a refrigerated compressed gas dryer which provides for varying therefrigerant temperature responsive to changes in the compressed gas loadon the refrigerant compressor.

[0003] Refrigerated compressed gas dryers are used to remove moistureand water vapor from compressed gas streams which are introduced throughthe gas compressor intake when the compressed gas is taken from theambient with its accompanying relative humidity. Once the gas iscompressed, its vapor holding capacity is reduced and the vaporcondenses into a liquid as the gas is cooled. Prior art typerefrigerated compressed gas dryers basically consist of apre-cooler/re-heater heat exchanger, an evaporator heat exchanger, aliquid separator and a liquid drain valve. The warm compressed gas ispassed through the pre-cooler/re-heater where it is cooled by theoutgoing cooled gas stream. As the warm compressed gas stream is cooledmoisture vapor begins to condense into a liquid. The compressed gasstream is then passed into the evaporator heat exchanger where it isfurther cooled to a lower temperature as the refrigerant evaporates atsome temperature below the desired temperature of the compressed gasstream exiting the evaporator. More water vapor is condensed into aliquid state in the evaporator and the cooled gas stream is passed tothe liquid separator where the condensed liquid is separated and removedfrom the system by the drain valve. The cooled and dried compressed gasstream is then returned through the pre-cooler/re-heater to pre-cool thewarm incoming compressed gas stream prior to being returned to thecompressed gas system piping. Compressed gas flow rates will vary as afunction of time in nearly every compressed gas dryer application. Theequipment can be expected to experience flows ranging from the maximumdesign flow rate down to a no-load, or zero, flow rate condition.

[0004] The refrigeration system of a typical refrigerated compressed gasdryer as described above basically consists of a refrigerant compressor,a refrigerant condenser, an expansion/restrictive device, and theevaporator described above. The temperature of the cooled compressedgas, as it exits the evaporator, defines the thermal performance ofcompressed gas dryers. This is typically expressed at the design flowrate. Increased cooling of the warm compressed gas results in lowerexiting evaporator compressed gas temperatures and higher levels ofmoisture removal. However, there is a practical limit to the amount ofcooling that can be done in the evaporator of a refrigerated gas dryer.Cooling the warm compressed gas stream down to a temperature below thefreezing point of water creates a situation where the condensate canfreeze and block the free path of the compressed gas stream, thus,increasing the pressure drop across the dryer. In extreme circumstances,the flow can be blocked completely, starving the downstream process ofcompressed gas. This failure situation will most likely occur duringcompressed gas flow rates that are much less than the maximum designflow rate. When using evaporators constructed from smooth tubing, thefreeze-up failure potential necessitates that the refrigeranttemperature in the evaporator be above the freezing point of water, andheld fixed and steady, as the load varies from no load to full load. Allmanufacturers of refrigerated compressed gas drying equipment mustaddress how to control the evaporator refrigerant temperature in orderto prevent condensate freeze-up under low or no load operatingconditions, while providing the thermal performance advertised at a fullload situation.

[0005] Presently, the most common method of controlling the evaporatorrefrigerant temperature in the compressed gas dryer is through the useof a hot gas by-pass valve, which is a pressure-regulating valve that isset to maintain a constant refrigerant pressure in the evaporator andrefrigerant compressor suction line. The by-pass valve operates bymetering high-pressure refrigerant discharge gas into the refrigerantcompressor suction line whenever the suction pressure drops below theset point of the pressure regulating by-pass valve. By understanding thesaturation temperature/pressure correlation of the refrigerant gas, theevaporator refrigerant temperature can be indirectly regulated bymaintaining a constant refrigerant suction pressure. Thistemperature/pressure correlation refers to the unique physicalsaturation properties of each refrigerant; that is, as a refrigerantchanges phase from a liquid to a vapor (i.e., boils or evaporates), itwill do so at a constant temperature and pressure. If the pressure iscontrolled and maintained while this phase change occurs, thetemperature is also maintained. Therefore, the more precisely thepressure is maintained, the more accurately the evaporator temperature:is held constant. A typical pressure setting for the by-pass valve wouldbe a refrigerant saturation pressure that corresponds to a saturationtemperature of approximately 35 degrees Fahrenheit. Placing theequivalent temperature setting slightly above the freezing point ofwater allows for a small factor of safety in the event of any valvesetting drift.

[0006] Another commonly used method to maintain a constant refrigerantsuction pressure is to install an automatic pressure valve (“APV”) inplace of the expansion/restrictive device and the hot gas by-pass valve.The APV maintains proper refrigerant suction pressure by meteringhigh-pressure liquid refrigerant into the inlet of the evaporator. TheAPV is typically inexpensive and inaccurate. Under no-load conditions,the liquid refrigerant may not be effectively converted into a gas inthe evaporator, which can result in a liquid flood-back condition at therefrigerant compressor suction, with potential compressor damage. Also,as the load is applied to the dryer, the refrigerant suction pressureoften increases, resulting in poor thermal performance. Some of thenewer technologies used to maintain a constant refrigerant suctionpressure include the use of variable speed refrigerant compressors whichoperate by altering the rotational speed, and therefore, the pumpingcapacity of the compressor. The refrigerant suction pressure can beincreased or decreased by decreasing or increasing, respectively, therotational speed of the compressor. Regardless of the manner ofcontrolling the suction pressure, typical prior art control schemesfunction to maintain a constant suction pressure, and thus a constantevaporator refrigerant temperature, regardless of the load on thecompressor. Consequently, prior art methods can suffer problems such aslower efficiency or freeze up conditions during compressor no-loadconditions.

[0007] Many conventional compressed gas dryers utilize smooth tubes inthe evaporator, which offer the advantage of a non-fouling surface thatperforms consistently throughout the life of the dryer. Other advantagesare reduced pressure drop and relatively inexpensive manufacturingcosts. A disadvantage of smooth tube technology is that a relativelylarge amount of heat exchange surface is necessary in order to achievethe desired thermal performance at the design full load condition. Thiscan be particularly challenging when considering the no-load and partialload freeze up concerns discussed previously, as well as the need tooperate the evaporator at 35 degrees Fahrenheit, offering a 4 degreeFahrenheit approach temperature. The efficient packaging of these dryerscan be inherently more difficult. Extended surface heat exchanger tubesare often used in order to make the evaporator more compact. Theexternally finned surface of such designs offer a temperature gradientbetween the refrigerant and the compressed gas stream. This gradient canpermit the refrigerant temperature to be less than the freezing point ofwater, without the danger of freeze-up. A reduced refrigeranttemperature results in a larger temperature approach, and less requiredsurface area. While the length required for this design is reduced ascompared to the smooth tube designs, the cost of the tube, and thedesign, can generally be greater. The designer may also have to addressthe concerns of excessive pressure drop.

[0008] Small, compact heat exchangers, such as brazed plate, or bar andframe type heat exchangers, offer an extremely attractive packagingsolution for a compressed gas dryer, but, again, can be much more costlythan the smooth tube designs. As these designs do not incorporateextended surfaces and the above discussed temperature gradients, therefrigerant temperatures must remain above the freezing point of waterin order to perform reliably under all operating conditions. A preciseand constant evaporator refrigerant temperature control is imperative tothese designs.

[0009] Due to the factors explained above, there has generally been nosingle optimum heat exchanger design for a compressed gas dryer. Aproblem has been that prior art designs are configured to maintain aconstant suction pressure, and thus evaporator refrigerant temperature,regardless of the compressed gas load on the refrigerant compressor.Consequently, there has been a compromise between the desired featuresof thermal performance, pressure drop performance, reliable operation,size, cost and packaging. The shortcomings of prior art refrigeratedcompressed gas systems described above illustrates the need for acontrol system for a refrigerated compressed gas dryer which can varythe evaporator refrigerant temperature in response to changes in theload on the refrigerant compressor. Consequently, the coolingcapability, per-unit length, of any given length heat exchanger can bemaximized.

SUMMARY

[0010] A variable evaporator control system and method are provided foradjusting the evaporator refrigerant temperature responsive to changesin the load on the refrigerant compressor in a refrigerated compressedgas dryer. A control system according to the invention can utilize, forexample, pressure and temperature sensors, a pair of temperaturesensors, or a single appropriately positioned temperature sensor. Eachof the sensors can be positioned at preselected locations in the systemto provide feedback to a processor which can analyze the output in orderto determine whether to increase or decrease the approach temperature,i.e., the difference between the temperature of the warm gas and therefrigerant temperature at the inlet of the heat exchanger. The controlsystem can preferably include at least one temperature sensor formonitoring the temperature of the refrigerant at the evaporator. Therefrigerant suction pressure can be controlled to vary the temperatureof the refrigerant at the evaporator inlet to generally maintain adesired outlet compressed gas temperature irrespective of the load onthe refrigerant compressor. In this way, the temperature of the driedcompressed gas exiting the evaporator is generally maintained whilemaking efficient use of the evaporator. For example, the evaporator canhave a shorter effective length and still provide the desired level ofcooling both at maximum design load for the evaporator and also duringlow or zero load on the refrigerant compressor. This can be accomplishedwhile avoiding potential freeze up problems which conventionally occurin systems which maintain a generally constant suction line pressureregardless of the load on the compressor. Moreover, this can beaccomplished using a smooth tube evaporator with all of the attendantadvantages while avoiding the potential freeze up problems which can beproblematic with smooth tube designs.

[0011] According to the invention, the refrigerant suction pressure canbe adjustably controlled in different ways, including, for example,using an electrically adjustable by-pass valve, varying the speed of avariable speed compressor, or using an unloading compressor arrangement.Adjustments in the refrigerant temperature at the inlet of the heatexchanger can be made generally in response to changes in the load onthe compressor. In particular, a lower refrigerant temperature can bemaintained where there is a high load on the compressor. However, as theload on the compressor decreases, the refrigerant temperature can beadjusted upwards, in order to avoid potential freeze up problems whichcould occur if the compressed gas temperature were reduced below thefreezing point of water. In a presently preferred embodiment,temperature can be sensed at a single point in the system wherein thetemperature is indicative of the load on the compressor. Feedback fromthis single point temperature sensor can be utilized to adjust theapproach temperature depending on the load on the compressor.

[0012] Other details, objects, and advantages of the invention willbecome apparent from the following detailed description and theaccompanying drawing FIGS. of certain embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein:

[0014]FIG. 1 illustrates diagrammatically a prior art refrigeratedcompressed gas dryer which uses a by-pass valve to regulate suctionpressure.

[0015]FIG. 2 illustrates diagrammatically a prior art refrigeratedcompressed gas dryer which uses an automatic pressure valve to regulatesuction pressure.

[0016]FIG. 3.1 graphically illustrates temperature profiles of prior artcompressed gas dryers, such as shown in FIGS. 1 and 2, employing a 4degree approach temperature.

[0017]FIG. 3.2 graphically illustrates temperature profiles of prior artcompressed gas dryers, similar to FIG. 3.1, except employing a 14 degreeapproach temperature.

[0018]FIG. 3.3 graphically illustrates temperature profiles of acompressed gas dryer utilizing a control system and method according tothe invention.

[0019]FIG. 4 illustrates diagrammatically a presently preferredembodiment of the invention using pressure and temperature sensors.

[0020]FIG. 5 illustrates diagrammatically an alternative embodiment ofthe invention using two temperature sensors.

[0021]FIG. 6 illustrates diagrammatically an alternative embodiment ofthe invention using a single temperature sensor.

[0022]FIG. 7 illustrates an embodiment of a smooth tube evaporatordesign and a presently preferred embodiment of an apparatus for a singlepoint temperature sensing control system and method for use with theembodiment of the invention shown in FIG. 6.

[0023]FIG. 8 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIG. 4 except using an unloadingcompressor.

[0024]FIG. 9 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIG. 5 except using an unloadingcompressor.

[0025]FIG. 10 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIG. 6 except using an unloadingcompressor.

[0026]FIG. 11 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIGS. 4 and 8, except using avariable speed compressor.

[0027]FIG. 12 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIGS. 5 and 9, except using avariable speed compressor.

[0028]FIG. 13 illustrates diagrammatically an alternative embodiment ofthe invention similar to that shown in FIGS. 6 and 10, except using avariable speed compressor.

DETAILED DESCRIPTION

[0029] Before describing the invention, a more detailed description ofprior art type refrigerated compressed gas dryers is provided for easeof understanding more clearly the advantages of the invention. Referringto FIG. 1, a prior art type refrigerated compressed gas dryer 15 isshown basically consisting of, as described in the background, apre-cooler/re-heater heat exchanger 21, a gas-to-refrigerant evaporatorheat exchanger 22, a liquid separator 23 and a single or multiple liquiddrain valve(s) 24. The incoming warm compressed gas 17, which containswater vapor, flows into the pre-cooler/re-heater 21, where it is cooledby the outgoing cold gas stream 19. The pre-cooler/re-heater 21 helps toreduce the heat load placed on the refrigeration system. As the warmcompressed gas stream 17 is cooled, the moisture vapor begins tocondense into a liquid. The compressed gas and condensed moisture thenleaves the pre-cooler section of the pre-cooler/re-heater 21 and entersthe compressed gas inlet 46 of the gas-to-refrigerant evaporator heatexchanger 22. Here the warm compressed gas stream 17 is additionallycooled to a lower temperature, usually 39 degrees Fahrenheit, as therefrigerant evaporates at some temperature (usually about 35 degreesFahrenheit) below the desired temperature of the compressed gas streamexiting the compressed gas outlet 48 of the evaporator 22. Again, morewater vapor is condensed into a liquid state. Exiting the evaporator 22,the cooled compressed gas stream 19 flows into the liquid separator 23,where the condensed liquid is separated from the cooled compressed gasstream 19. After separation, this liquid is collected and removed fromthe system entirely by one, or many, liquid drain valves 24. The cooled,and liquid-free compressed gas stream 19 then exits the separator andre-enters the pre-cooler/re-heater 21. Here the cooled compressed gasstream 19 is re-heated by transferring heat with the warm incomingcompressed gas stream 17. The reheated compressed gas stream 19 thenexits the dryer 15 and continues flowing through the compressed gassystem piping (not shown). Compressed gas flow rates will vary as afunction of time in nearly every compressed gas dryer application. Theequipment can be expected to experience flows ranging from the maximumdesign flow rate down to a no load, or zero flow rate, condition.

[0030] The refrigeration system 16 in a typical compressed gas dryer 15is also shown in FIG. 1. In its basic form, it consists of arefrigeration compressor 25, a refrigerant condenser 26, anexpansion/restrictive device 27, and the gas-to-refrigerant evaporatorheat exchanger 22 described above. The restrictive devices 27 mayinclude capillary tubes, thermal expansion valves (TXV), orifices,electronic expansion valves, and other devices known in the art. Thetemperature of the cooled compressed gas 19, as it exits the evaporator22, defines the thermal performance required of compressed gas dryers.This is typically expressed at the design flow rate. Increased coolingof the warm compressed gas 17 results in lower exiting evaporator 22compressed gas temperatures and higher levels of moisture removal.However, there is a practical limit to the amount of cooling that can bedone in the evaporator 22 of a refrigerated gas dryer. Cooling the warmcompressed gas stream 17 down to a temperature below the freezing pointof water creates a situation where the condensate can freeze and blockthe free path of the compressed gas stream, thus, increasing thepressure drop across the dryer 15. In extreme circumstances, the flowcan be blocked completely, starving the downstream process of compressedgas. This failure situation will most likely occur during compressed gasflow rates that are much less than the maximum design flow rate. Whenusing evaporators constructed from smooth tubing, the freeze-up failurepotential necessitates that the refrigerant temperature in theevaporator 22 be above the freezing point of water, and held fixed andsteady, as the load varies from no load to full load. All manufacturersof refrigerated compressed gas drying equipment must address how tocontrol the evaporator 22 refrigerant temperature in order to preventcondensate freeze-up under low or no load operating conditions, whileproviding the thermal performance advertised at a full load situation. Adescription of some of these methods follows.

[0031] As explained above in the background section, some prior artmethods of controlling the evaporator 22 refrigerant temperature do soindirectly by maintaining a generally constant pressure in theevaporator 22 and suction line 31. The pressure being maintained in thesuction line 31 generally corresponds to a predetermined refrigerantpressure to be provided at the evaporator. The most common manner ofregulating the suction pressure is using a hot gas by-pass valve 28. Theby-pass valve 28 is a pressure-regulating valve that can be set tomaintain a constant refrigerant suction pressure. The by-pass valve 28meters high-pressure refrigerant discharge gas into the suction line 31whenever the suction pressure drops below the set point of the by-passvalve 28. By understanding the saturation temperature/pressurecorrelation of the refrigerant gas, the evaporator 22 refrigeranttemperature is indirectly controlled by maintaining a constantrefrigerant suction pressure. The more precisely the suction pressure ismaintained constant, the more accurately a constant evaporator 22temperature is held. A typical pressure setting for the by-pass valve 28would be a refrigerant saturation pressure that corresponds to 35degrees Fahrenheit. Placing the equivalent temperature setting above thefreezing point of water allows for a small factor of safety in the eventof any valve setting drift. FIG. 1 shows the inlet 33 of the by-passvalve 28 connected to the discharge line 36 of the compressor 25 and theoutlet 39 of the by-pass valve 28 feeding into the suction line 31. Thisis the most common method, however, many manufacturers choose to feedthe outlet 39 of the hot gas by-pass valve 28 into the inlet 41 of theevaporator 22.

[0032] Another commonly used method to maintain a constant refrigerantsuction pressure is to replace the expansion/restrictive device 27 andhot gas by-pass valve 28 with an automatic pressure valve (“APV”) 29, asshown in FIG. 2. The APV 29 maintains a constant refrigerant suctionpressure by metering high-pressure liquid refrigerant into the inlet 41of the evaporator 22. The APV 29 is typically inexpensive, but nothighly accurate. Under no-load conditions, the liquid refrigerant maynot be effectively converted into a gas in the evaporator 22, which canresult in a liquid flood-back condition at the compressor suction, withpotential subsequent compressor 25 damage. Additionally, as the load isapplied to the dryer 15, the refrigerant suction pressure oftenincreases resulting in poor thermal performance.

[0033] A more recent technology used to maintain a constant refrigerantsuction pressure is the use of variable speed refrigerant compressorswhich operate by altering the rotational speed, and therefore, thepumping capacity of the compressor. To maintain a constant refrigerantsuction pressure, the rotational speed of the compressor can beincreased or decreased, to decrease or increase, respectively, thesuction pressure. However, this design can require the use of powerfrequency inverters, suction line pressure sensors and/or temperaturesensors.

[0034] Referring now to FIGS. 3 through 13 generally, a variableevaporator control (“VEC”) system and method for a refrigeratedcompressed gas dryer can be provided, according to the invention, basedon an understanding of evaporator performance as discussed previously:that is, under a full load condition, there is a need for either a largetemperature difference between the refrigerant and the outlet gas (andless required heat exchanger surface), or a large amount of low costsurface (and a precisely controlled refrigerant temperature), in orderto have a cost effective, thermally performing design. In addition,during periods of light load, or no load, the refrigerant temperature ina smooth tube-type evaporator must be accurately and constantlymaintained above the freezing point of water. In order to satisfy bothof these design constraints, a refrigerant control system can beprovided having sufficient intelligence to control, or vary, therefrigerant temperature as compressed gas loads are applied to andremoved from the dryer.

[0035]FIGS. 3.1 through 3.3 graphically illustrate the design criteriadescribed above and highlight the advantages of a VEC system forcontrolling, e.g., varying, the evaporator refrigerant temperature. FIG.3.1 shows the temperature profiles in the evaporator 22 of a typicalrefrigerated compressed gas dryer 15 as a function of the“characteristic length” of the evaporator 22. The graph assumes anevaporator 22 refrigerant temperature of 35 degrees Fahrenheit that isheld constant under all load conditions. A slight amount of refrigerantsuperheat is present at the refrigerant outlet 43 of the evaporator 22.This superheat is shown as 5 degrees Fahrenheit, and equates to arefrigerant exit temperature of 40 degrees Fahrenheit. Under a fulldesign flow condition, the warm compressed gas stream 17 is cooled froma temperature of 70 degrees Fahrenheit at the compressed gas inlet 46 ofthe evaporator 22 down to a temperature of 39 degrees Fahrenheit at thecompressed gas outlet 48 of the evaporator 22. Heat exchange occursalong the entire length of the evaporator 22. The length of theevaporator 22 in this case is defined, for comparative purposes, as 1.0(no unit designation). The approach temperature is 4 degrees Fahrenheit.The approach temperature is the difference between the refrigerant (atthe refrigerant inlet 41) and the compressed gas (at the compressed gasoutlet 48). The dotted line depicts the temperature profile along thelength of the evaporator 22 during periods of light load. Note that theapproach temperature remains nearly the same as the full load case, andthat the entire length of the evaporator 22 is not needed when reducedflows are present. This can be typical of current designs.

[0036]FIG. 3.2 shows similar information in type, but with a design thatuses a lower evaporator 22 refrigerant temperature of 25 degreesFahrenheit, and a required evaporator 22 characteristic length ofapproximately 0.5. The temperature of the cooled compressed gas stream119, at the compressed gas outlet 48, remains at 39 degrees Fahrenheitand the approach temperature at the outlet 48 is 14 degrees Fahrenheit.This is an acceptable design for a full load condition. However, withcurrent control technology and methods, the evaporator 22 refrigeranttemperature would remain constant at 25 degrees Fahrenheit as the loadis reduced., or removed. Consequently, if the compressed gas load dropsto the light load condition, the compressed gas stream will be cooled toa temperature below the freezing point of water, as the approachtemperature nears 4 degrees Fahrenheit. This can lead to a detrimentalcondensate freeze-up condition.

[0037]FIG. 3.3 illustrates operating characteristics which can beobtained according to invention. Since the evaporator 22 refrigeranttemperature is readjusted as the load fluctuates from full design loadto light and no load, the approach temperature is also readjusted, from14 degrees Fahrenheit under the full load condition, to 4 degreesFahrenheit during the light load case. Therefore, the characteristiclength of the evaporator 22 can be optimally sized for the full loadcondition (a 14 degree Fahrenheit approach), resulting in acharacteristic length which is approximately 50% that of conventionaldesigns. According to the invention, the VEC system can cause therefrigerant temperature to rise as the load is reduced, thus maintaininga constant compressed gas exit temperature. Ultimately, the evaporator22 refrigerant temperature is brought above the freezing point of water,thereby safely eliminating the concern of condensate freeze-up.

[0038] One aspect of as presently preferred control method/system iscontrolling the cooling process using feedback from one or moretemperature and/or pressure sensors which can indicate the load on thecompressor. However, such sensors are not being used simply to maintaina constant refrigerant temperature as in the prior art. Rather, thesensors can be used to implement increased control over the system bymonitoring and adjusting the refrigerant temperature, and thus theapproach temperature, in the evaporator in order to generally maintainthe cooled gas exit temperature at a desired value. The prevailing loadon the compressor at a given time can be indirectly indicated by thefeedback from the sensors, and the temperature of the refrigerant can beadjusted accordingly, thereby adjusting the approach temperature, toavoid a potential freeze up condition at light or zero loads. Thus, byanalyzing the temperature sensor feedback, the microprocessor candetermine the compressor load, although, as explained above, themicroprocessor does not directly determine the magnitude of thecompressor load. Rather, by using the temperature sensor(s) properly andunderstanding the characteristics of the cooling system, there is noneed to know the load on the compressor. The compressed gas exittemperature will be generally maintained regardless of the compressorload.

[0039] Consequently, it can be understood that the cooling capabilityper-unit-length of a heat exchanger of any given fixed length can bemaximized by removing the conventional restriction of maintaining aconstant approach temperature irrespective of the load on thecompressor. The approach temperature can be large, i.e., the refrigeranttemperature low, when the load on the compressor is above a certainlevel, thus maximizing the amount of cooling possible for a given lengthheat exchanger. However, when the load on the compressor reduces below acertain level, a smaller approach temperature can be implemented, byincreasing the refrigerant temperature above freezing, to eliminate anypotential for freeze up.

[0040] A benefit of a VEC system according to the invention is that alow cost, smooth tube evaporator can successfully be utilized with largetemperature differences between the refrigerant and the compressed gasin order to satisfy the desired thermal and pressure drop performancecriteria while eliminating the potential of condensate freeze-up duringthe light load and no load conditions.

[0041] Hereinafter, will be described in detail certain presentlypreferred embodiments of VEC systems for refrigerated compressed gasdrying applications. The following description of certain embodiments,as illustrated in FIGS. 4 through 13, are not intended to be exhaustive,but only representative of embodiments of VEC systems according to theinvention which can employ currently available supporting technology.

[0042] VEC Systems Utilizing an Electronic By-Pass Valve

[0043] As previously discussed, the evaporator 22 refrigeranttemperature can be controlled indirectly though the control of therefrigerant suction pressure. Traditional technologies haveconventionally utilized mechanical pressure regulating valves, such asthe hot gas by-pass valve 28, which can be manually set to maintain aconstant suction pressure. However, a new technology has emerged whichplaces a small adjustment motor, e.g., a stepper motor, on to the hotgas by-pass valve 28 body, such as the motor 51 shown in FIG. 5, toprovide for electronic control of the by-pass valve 28. Electronic hotgas by-pass valves are available manufacturers such as Sporlan ValveCompany, headquartered in Washington, Mo. Consequently, by electronicmeans, the setting of this motor, and thus the by-pass valve 28 can beadjusted as required. Using proper sensing techniques and microprocessorintelligence, the evaporator refrigerant temperature can be adjusted byadjusting the suction pressure. In particular, using the motor 51operated by-pass valve 28, the suction pressure can be increased toraise the evaporator refrigerant temperature above the freezing point ofwater when compressed gas loads are removed. The evaporator refrigeranttemperature can be measured at the evaporator inlet. Conversely, thesuction pressure can be decreased to lower the evaporator refrigeranttemperature as the load on the compressor is increased, simply bycontrolling the stepper motor 51 on the by-pass valve 28. Such aelectronically controllable by-pass valve can also be implemented withmany of the known refrigerant expansion/metering valves which controlthe refrigerant flow into the evaporator, such as a capillary tube, athermal expansion valve, an electronic expansion valve, or an orifice.

[0044] Sensing Techniques

[0045] Some sensing techniques which can be employed when using anelectronically controllable hot gas by-pass valve 28 as part of a VECsystem are described below.

[0046] A. Suction Pressure/Compressed Gas Temperature

[0047] A presently preferred embodiment of a VEC system 100 utilizing aby-pass valve 28 which is controllable electronically via motor 51 isshown in FIG. 4. As shown, a pressure sensor 54 can be used to monitorthe refrigerant suction pressure in the suction line 31, and atemperature sensor 57 can be used to monitor the compressed gastemperature at the compressed gas outlet 48 of the evaporator 22. Outputfrom the pressure sensor 54 and temperature sensor 57 can be supplied toa processor 60, such as a microprocessor, which can evaluate theinformation and determine any required adjustments to be made to theby-pass valve 28 via motor 51 as the load on the refrigerant compressor25 either increases or decreases. The load on the refrigerant compressor25 can vary due to changes in either the volume or the temperature ofthe warm compressed gas 17 circulated through the evaporator 22.Specifically, the temperature of the compressed gas stream at the outlet48 of the evaporator 22 can be monitored to generally maintain thistemperature at a desired level. Since the compressed gas exittemperature can change if the load on the gas compressor changes,because the suction pressure is maintained constant by the by-pass valve28, the compressed gas exit temperature can be utilized to adjust thesuction pressure using the motor 51 in order to maintain the compressedgas exit temperature at the desired value. This can maximize theefficiency of the system and eliminate potential freeze up problems.

[0048] B. Refrigerant Temperature/Compressed Gas Temperature

[0049] Referring to FIG. 5, another embodiment of a VEC system 105utilizing a hot gas by-pass valve 25 controllable electronically viamotor 51 is shown. In this embodiment, instead of a pressure sensor onthe suction line 31, a first temperature sensor 63 can be used tomonitor the refrigerant temperature at the evaporator 22 inlet 41. Asecond temperature sensor 66 can be used to monitor the compressed gastemperature at the compressed gas stream outlet 48 of the evaporator 22.As explained above, this information can be supplied to themicroprocessor 60 which can evaluate the information to determine therequired adjustment to be made to the by-pass valve 28 via the motor 51as the load on the compressor 25 increases or decreases.

[0050] C. Single Point Temperature

[0051] A further embodiment of a VEC system 110 is shown in FIG. 6,wherein the system can utilize a single point temperature sensingmethod. This method can require determining an optimum sensing locationfor a single temperature sensor 69 which can provide the temperature ofthe compressed gas during periods of actual gas flow and also provide anaccurate evaporator 22 refrigerant temperature during periods of noflow. The microprocessor 60 can be supplied with this information andutilize it to determine the required adjustment to be made to theelectronically controllable by-pass valve 28 via motor 51 as the load onthe compressor 25 is increased or decreased.

[0052]FIG. 7 illustrates a particular embodiment of the single pointtemperature sensing method shown in FIG. 6, depicting an optimumlocation for, and presently preferred embodiment of, a single pointtemperature sensor 69. The evaporator 22 can be of a design utilizing amultiple smooth tube bundle 72 a-72 e enclosed in a single cover shell75. Compressed gas 18 flows through the tubes 72 a-72 e and therefrigerant 77 resides inside the cover shell 75. The end of the tubebundle 72 a-72 e can be isolated from the cover shell 75 with, forexample, a simple brazed tube sheet 78. The flow pattern is showncounter-flow, with the refrigerant 77 entering the evaporator 22 abovethe tube sheet 78 located near the compressed gas outlet tubes 72 a-72e. The refrigerant 77 exits the evaporator 22 at the opposite end of theevaporator 22, near the compressed gas inlet. As mentioned above, it canbe necessary to determine a physical location for the temperature sensor69 whereby the sensed temperature would be indicative of the compressedgas stream 18 temperature during periods of full and light flow, yetalso indicative of the refrigerant 77 temperature in the evaporator 22during a no load situation. Simply placing the temperature sensor 69directly in the gas stream 18 can satisfy the initial constraint quitewell, but when gas flow ceases, the temperature could rise in thestagnant gas environment, forcing the microprocessor 60 to lower therefrigerant 77 temperature. This result is opposite of the desiredeffect and can lead to a freeze-up condition. Conversely, by placing thetemperature sensor 69 directly in the refrigerant 77, or on the tubesheet 78, the temperature sensor 69 may respond appropriately during theno load condition, but may not behave correctly as a load is applied. Infact, the refrigerant 77 temperature may simply remain constant underall conditions.

[0053] Moreover, as further shown in FIG. 7, a presently preferredsolution for implementing a VEC system using single point temperaturesensing can include placing a thermally conductive extension 80 on theend of one of the smooth tubes, e.g., tube 72 d, and then determiningthe appropriate temperature sensor 69 position (labeled as “x”) whichcan accurately indicate the compressed gas stream 18 temperature when alight to full flow is present (a combination of conductive andconvective heat transfer). However, if placed too close to the tubesheet 78, the temperature reading could be biased by conductive heattransfer into the refrigerant 77. Thus, the solution can further includeinserting a temperature probe 83 through the wall of the cover shell 75and assuring proper thermal contact with the outside surface of theextended tube, for example tube 72 d. In this manner, during a verylight or zero flow condition, a purely conductive heat transfer path canbe established with the evaporating refrigerant 77 above the tube sheet78.

[0054] As a result, this solution can provide accurate temperatureinformation permitting control over the system under all conditions byfacilitating an indication of the load, i.e., volume of warm compressedgas 17 being circulated through the evaporator 22. By knowing the volumeof compressed gas being circulated, i.e., full or light load conditions,the approach temperature can be adjusted accordingly to enable maximumcooling for an evaporator 22 of any given length. For example, asillustrated in the graphs in FIGS. 3.1 through 3.3, a larger approachtemperature, i.e., a lower refrigerant inlet temperature, can beimplemented during a full load condition with no potential for freezeup. Conversely, a smaller approach temperature, i.e., a higherrefrigerant inlet temperature, can be provided during a light loadcondition to avoid potential freeze up.

[0055] A housing 84 can be provided through the cover shell 75 to theextension 80, in which the temperature sensor 83 can be housed. Testinghas indicated that, using approximately 0.25 inch (outer diameter)smooth tubes 72 a-72 d, the proper distance, “x,” from the tube sheet 78can be about 0.25 inch. This distance has been satisfactory for variousnumbers of the smooth tubes 72 a-72 d, and different diameter covershells 75.

[0056] In sum, the temperature of the compressed gas at the outlet 48 ofthe evaporator 22 dominates the sensor 83 reading when there is a lightto heavy load on the compressor, and the refrigerant temperaturedominates when there is a very light to zero load. Thus, a single-pointtemperature sensor, when placed in a proper location, can providesufficient feedback to the microprocessor to control the cooling systemregardless of the flow condition, i.e., the volume of warm compressedgas being circulated through the evaporator 22. For example, thecompressed gas exit temperature can be set at 37 or 38 degreesFahrenheit. If the compressed gas exit temperature increases, therefrigerant temperature will be permitted to drop until the 37 degreeFahrenheit temperature is satisfied. This is accomplished with no dangerof freeze up because the temperature of the compressed gas is stillbeing maintained above freezing even though the refrigerant temperaturemay fall below freezing at that set point. Then, if the compressor loaddrops off, the temperature detected by the sensor 83 will be dominatedby the temperature of the refrigerant, due to the conductive heattransfer path directly from the refrigerant. If the compressor load issignificantly reduced, the temperature of the compressed gas no longerdominates the temperature sensor 83; the refrigerant temperature nowdominates it. However, since the set point is maintained at about 37degrees Fahrenheit, the refrigerant temperature is permitted to riseabove the freezing point of water. When using multiple sensors, theprocessor may also be programmed with the appropriate logic andcomparative information between the two temperatures, i.e., compressedgas temperature versus refrigerant temperature, to properly control therefrigerant temperature.

[0057] VEC Systems Utilizing an Unloading-type Compressor

[0058] Another presently preferred embodiment of a VEC system can relyon varying the capacity of the compressor to control the refrigerantsuction pressure, and corresponding evaporator refrigerant temperature.This can be realized through the use of unloading-type refrigerantcompressors. Whenever a lower suction pressure (lower evaporatorrefrigerant temperature) is desired, the capacity of the compressor canbe increased; conversely, as the need for increasing suction pressure(higher evaporator refrigerant temperature) is detected, the compressorcapacity can be decreased. This capacity control can be achieveddiscretely (i.e., fill capacity or no capacity) in some compressordesigns, such as the digital, or unloading, scroll compressor. Othermodels of multi-cylinder reciprocating compressors are designed topermit levels of capacity reduction, or capacity addition, in steps.Using various sensing techniques and the proper microprocessorintelligence, the suction pressure can therefore be raised as compressedgas loads are removed, or lowered as the load increases, by activatingthese unloading and loading mechanisms.

[0059] Sensing Techniques

[0060] Some sensing techniques which can be employed when usingunloading type compressors as part of a VEC system are described below.

[0061] A. Suction Pressure/Compressed Gas Temperature

[0062] A presently preferred embodiment of a VEC system 115 utilizing anunloading type compressor 86 is shown in FIG. 8. In this embodiment,similarly to the embodiment shown in FIG. 4, pressure sensor 54 can beused to monitor the refrigerant suction pressure at the suction line 31and temperature sensor 57 can be used to monitor the compressed gastemperature at the compressed gas outlet 48 of the evaporator 22. Themicroprocessor 60 can receive and evaluate this information to determinewhen to load or unload the compressor 86 in order to adjust the suctionpressure, and thus the evaporator 22 refrigerant temperature, as thecompressor 86 load is increased and decreased.

[0063] B. Refrigerant Temperature/Compressed Gas Temperature

[0064] Referring to FIG. 9, another embodiment of a VEC system 120utilizing an unloading type compressor 86 is shown wherein, similarly toFIG. 5, first temperature sensor 63 can be used to monitor therefrigerant temperature at the inlet 41 of the evaporator 22 and secondtemperature sensor 66 can be used to monitor the compressed gastemperature at the evaporator 22 compressed gas stream outlet 48. Themicroprocessor 60 then receives and utilizes this information todetermine when to load or unload the compressor 86 in order to adjustthe suction pressure as the compressor 86 load is increased ordecreased.

[0065] C. Single Point Temperature

[0066] Similarly to FIG. 6, FIG. 10 illustrates an embodiment of a VECsystem 125 utilizing an unloading type compressor 86 in a single pointtemperature sensing method. As explained previously, this method canrequire determining an optimum sensing location for the singletemperature sensor 69 which can accurately indicate both the temperatureof the compressed gas during periods of actual gas flow, and theevaporator 22 refrigerant temperature during periods of no flow. Themicroprocessor 60 can be supplied with this information which isevaluated to determine when to load or unload the compressor 86 in orderto adjust the suction pressure as the load on the compressor 86 isincreased or decreased. For details regarding locating an appropriatetemperature sensing location for the temperature sensor 69, refer to thedescription provided in connection with FIG. 7.

[0067] VEC Systems Utilizing a Variable Speed Compressor

[0068] As noted earlier, variable speed refrigerant compressors areavailable which can vary the refrigeration capacity by altering therotational speed of the compressor. This type of compressor can also beutilized in embodiments of a VEC system as a means to change therefrigerant suction pressure as compressed gas loads are applied to andremoved from the dryer. To increase the refrigerant suction pressure,the speed of the compressor can be decreased; to decrease therefrigerant suction pressure, the speed can be increased. Using varioussensing techniques and the proper microprocessor intelligence, thesuction pressure can be raised as compressed gas loads are removed, andlowered as compressed gas loads increase, by controlling the rotationalspeed of the compressor.

[0069] Sensing Techniques

[0070] Some sensing techniques which can be employed when using variablespeed compressors as part of a VEC system are described below.

[0071] A. Suction Pressure/Compressed Gas Temperature

[0072]FIG. 11 illustrates an embodiment of a VEC system 130 utilizing avariable speed compressor 90. In this embodiment, similarly to theembodiments of the invention shown in FIGS. 4 and 8, pressure sensor 54can monitor the refrigerant suction pressure at the suction line 31 andtemperature sensor 57 can monitor the compressed gas temperature at theoutlet 48 evaporator 22. This information can be supplied to themicroprocessor 60 which can evaluate the information and determinewhether to increase or decrease the rotational speed of the compressor90 to adjust the suction pressure, and thus the evaporator refrigeranttemperature, as the compressed gas load on the compressor 90 isincreased or decreased.

[0073] B. Refrigerant Temperature/Compressed Gas Temperature

[0074] Similarly to FIGS. 5 and 9, another embodiment of a VEC system135 using a variable speed compressor 90 is shown in FIG. 12. In thisVEC system 135, first temperature sensor 63 can monitor the evaporator22 refrigerant temperature at the inlet 41 to the evaporator 22 andsecond temperature sensor 66 can monitor the compressed gas temperatureat outlet 48 of the evaporator 22. This information can be supplied tothe microprocessor 60 for use in determining whether to increase ordecrease the rotational speed of the refrigerant compressor 90 in orderto adjust the suction pressure as the compressed gas load on thecompressor 90 is increased or decreased.

[0075] C. Single Point Temperature

[0076] Similarly to FIGS. 6 and 10, FIG. 13 illustrates a furtherembodiment of a VEC system 140 using a single point temperature sensingmethod. As explained previously, this method can require determining anoptimum location for the single temperature sensor 69 which will beindicative of the compressed gas temperature during periods of actualgas flow, and will also provide an accurate refrigerant evaporator 22refrigerant temperature during periods of no flow. The microprocessor 60can be provided with this information for use in and determining whetherto increase or decrease the rotational speed of the refrigerantcompressor 90 in order to adjust the suction pressure as the load on thecompressor 90 is increased or decreased. For details regarding locatingan appropriate temperature sensing location, refer to the descriptionprovided in connection with FIG. 7.

[0077] As can be understood from the preceding description of certainembodiments of the invention, such a control system and method canpermit the use of smaller evaporators in conjunction with compressed gasdryers, which provides more efficient packaging, lower manufacturingcosts, and reduced pressure drop. Using the control system with smoothtube evaporator designs also permits non-fouling heat exchangeperformance, lower manufacturing costs and reduced pressure drop. Thecontrol system thus permits the use of compact heat exchanger designsemploying refrigerant temperatures below the freezing point of water(plate heat exchangers, bar and frame heat exchangers, etc.) without thedanger of condensate freeze-up at light load and no load conditions byadjusting the approach temperature according to changes in the warmcompressed gas load. Since the control system can respond to the actualcompressed gas temperature, proper and constant dryer performance andmoisture removal at all flow rates and conditions can be assured. Thecontrol system can also be embodied in many of the current technologiesavailable for refrigerant evaporator pressure/temperature control. Thesetechnologies may exist as control components, e.g., control valves, oras integral systems contained in the refrigerant compressors, such asunloading mechanisms, variable speed models, and the like.

[0078] Moreover, those of skill in the art will recognize that such acontrol system according to the invention can also be adapted forapplications in other areas of refrigeration and cooling. Accordingly,although certain embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodification to those details could be developed in light of the overallteaching of the disclosure. Therefore, the particular embodimentsdisclosed herein are intended to be illustrative only and not limitingto the scope of the invention which should be awarded the full breadthof the following claims and any and all embodiments thereof.

What is claimed is:
 1. A refrigerated compressed gas dryer variable evaporator control method comprising: a. introducing compressed gas at a first temperature through a first flow path in a heat exchanger; b. circulating a refrigerant through a second flow path in said heat exchanger, said refrigerant having a second temperature measured at a refrigerant inlet of said heat exchanger, said second temperature being less than said first temperature; c. cooling said compressed gas to a third temperature as it flows through said heat exchanger; and d. controlling said second temperature responsive to changes in said third temperature to generally maintain said third temperature at a desired value.
 2. The method of claim 1 further comprising: a. sensing a fourth temperature at a single location in said heat exchanger at which said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below said certain level; and b. controlling said second temperature responsive to changes in said fourth temperature to generally maintain said fourth temperature at said desired value.
 3. The method of claim 1 wherein said refrigerant is circulated under pressure, said method further comprising: a. maintaining said refrigerant at a predetermined pressure during circulation, said second temperature having a correlation to said predetermined pressure; and b. controlling said predetermined pressure responsive to changes in said third temperature such that said second temperature is correspondingly controlled to generally maintain said third temperature at said desired value.
 4. The method of claim 3 further comprising: a. sensing a fourth temperature at a single location in said heat exchanger at which said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below a certain level; and b. controlling said predetermined pressure responsive to changes in said third temperature such that said second temperature is correspondingly controlled to generally maintain said third temperature at said desired value.
 5. A variable evaporator control method wherein gas at a first temperature is passed through first flow path in a heat exchanger having a second flow path through which a refrigerant at a second temperature lower than said first temperature is circulated to cool said gas to a third temperature, said method comprising: a. sensing a fourth temperature at a single location in said heat exchanger at which said fourth temperature is representative of said third temperature when a gas load is above a certain level, and representative of said second temperature when said gas load is below said certain level; and b. adjusting said second temperature responsive to changes in said fourth temperature to generally maintain said fourth temperature at a desired value.
 6. The method of claim 5 further comprising: a. maintaining said refrigerant at a predetermined pressure during circulation, said second temperature having a correlation to said predetermined pressure; and b. controlling said predetermined pressure responsive to changes in said fourth temperature such that said second temperature is correspondingly controlled to generally maintain said fourth temperature at said desired value.
 7. A variable evaporator control refrigerated compressed gas dryer comprising: a. an evaporator heat exchanger having a first flow path for compressed gas at a first temperature; b. said evaporator heat exchanger having a second flow path for a refrigerant at a second temperature which is lower than said first temperature to cool said compressed gas to a third temperature; c. a compressor to circulate said refrigerant at a predetermined pressure, said predetermined pressure having a correlation to said second temperature; d. a temperature sensor to sense said third temperature at an outlet of said first flow path; e. a valve intermediate said second flow path and a source of refrigerant, said valve controllable to admit refrigerant into circulation to adjust said predetermined pressure; and f. a controller connected to receive feedback from said temperature sensor indicative of said third temperature, said controller controlling said valve to adjust said predetermined pressure and thus adjust said second temperature to generally maintain said third temperature at a desired value.
 8. The compressed gas dryer of claim 7 further comprising a second temperature sensor to sense said second temperature at an inlet of said second flow path.
 9. The compressed gas dryer of claim 7 further comprising a pressure sensor to sense said predetermined pressure in said second flow path and to provide feedback indicative of said predetermined pressure to said controller to control said valve to adjust said predetermined pressure and thus said second temperature to generally maintain said third temperature at said desired value.
 10. The compressed gas dryer of claim 7 further comprising a. said temperature sensor positioned at a single location to sense a fourth temperature wherein said fourth temperature is representative of said third temperature when a compressed gas load is above a certain level, and representative of said second temperature when said compressed gas load is below said certain level; and b. said controller connected to receive feedback from said temperature sensor indicative of said fourth temperature to control said valve to adjust said predetermined pressure and thus said second temperature such that said fourth temperature is generally maintained at said desired value.
 11. The compressed gas dryer of claim 10 wherein said evaporator heat exchanger further comprises a smooth tube evaporator heat exchanger.
 12. The compressed gas dryer of claim 11 wherein said smooth tube evaporator heat exchanger further comprises: a. an outer cover; b. a plurality of smooth tubes enclosed within said outer cover, said first flow path being at least partially through said plurality of smooth tubes; c. said second flow path being within said outer cover and communicating with an exterior of said plurality of smooth tubes; d. a tube sheet dividing a portion of said evaporator heat exchanger, said second flow path communicating on a first side of said tube sheet; e. each of said plurality of smooth tubes having an end extending through said tube sheet to a second side thereof which is not in communication with said second flow path; and f. said single point temperature sensor being disposed in contact with said end of at least one of said plurality of smooth tubes which extends through said tube sheet.
 13. The compressed gas dryer of claim 12 further comprising said end in contact with said single point temperature sensor having an extended length portion, and said single point temperature sensor disposed in contact with said extended length portion.
 14. A variable evaporator control system comprising: a. an evaporator heat exchanger having a first flow path for a gas at a first temperature and a second flow path for circulation of a refrigerant at a second temperature which is lower than said first temperature to cool said gas to a third temperature; b. a compressor to circulate said refrigerant; c. a temperature sensor positioned in said evaporator heat exchanger at a single location to sense a fourth temperature which is representative of said third temperature when a gas load is above a certain level, and representative of said second temperature when said gas load is below said certain level; and d. a controller connected to receive feedback from said temperature sensor indicative of said fourth temperature to control said second temperature to generally maintain said fourth temperature at a desired value.
 15. The variable evaporator control system of claim 14 further comprising: a. a compressor to circulate said refrigerant at a predetermined pressure, said second temperature having a correlation to said predetermined pressure; and b. a valve positioned intermediate said second flow path and a source of refrigerant, said valve controllable by said controller to adjust admission of said refrigerant into circulation to adjust said predetermined pressure and thus adjust said second temperature to generally maintain said fourth temperature at said desired value.
 16. A variable evaporator control method maximizing cooling of a gas passed through an evaporator heat exchanger of a given length, wherein gas at a first temperature flows through said evaporator heat exchanger in a first flow path and a refrigerant at a second temperature lower than said first temperature is circulated through said evaporator heat exchanger in a second flow path to cool said gas to a third temperature, the difference between said first and second temperatures being an approach temperature, said control method comprising: a. increasing said approach temperature as a gas load through said evaporator heat exchanger increases; b. decreasing said approach temperature as a gas load through said evaporator heat exchanger decreases; and c. wherein said third temperature is thus generally maintained at a desired value irrespective of said gas load.
 17. The method of claim 16 further comprising controlling said second temperature to implement said increasing or decreasing of said approach temperature.
 18. The method of claim 17 further comprising: a. sensing a fourth temperature at a single location in said evaporator heat exchanger at which said fourth temperature is representative of said third temperature when said gas load is above a certain level, and representative of said second temperature when said gas load is below said certain level; and b. controlling said second temperature responsive to said fourth temperature such that said fourth temperature is generally maintained at a desired value irrespective of said gas load.
 19. A compressed gas dryer having a heat exchanger with a first flow path for a compressed gas and a second flow path for a refrigerant to cool said compressed gas, the compressed gas dryer comprising: a. a temperature sensor to sense a temperature of said compressed gas at an outlet of said first flow path; and b. a controller connected to receive feedback from said temperature sensor indicative of said temperature of said compressed gas at said outlet of said first flow path, said controller controlling a temperature of said refrigerant at an inlet of said second flow path to generally maintain said temperature of said compressed gas at said outlet of said first flow path at a desired value.
 20. The compressed gas dryer of claim 19 further comprising: a. said temperature sensor of said compressed gas at said outlet of said first flow path being positioned in said heat exchanger at a single location; b. a temperature sensed at said single location being representative of said temperature of said compressed gas at said outlet of said first flow path when a compressed gas load is above a certain level; c. said temperature sensed at said single location being representative of said temperature of said refrigerant at said inlet of said second flow path when said compressed gas load is below said certain level; and d. said controller controlling said temperature of said refrigerant at said inlet of said second flow path to generally maintaining said temperature sensed at said single location at said desired value.
 21. The compressed gas dryer of claim 20 wherein said desired value further comprises a value above 32 degrees Fahrenheit. 